It is critical for users to have the flexibility to participate in a multimedia conference (i.e., involving voice, video and data) and to be able to support point-to-point and multipoint configurations using an ATM wide area network based H.321 multipoint control unit (MCU). The MCU provides bridging for audio, video, and/or data for multipoint calls. Recently, the ITU developed Recommendation H.321 for multimedia conferencing in Broadband Integrated Services Digital Network (B-ISDN) environments. The H.321 recommendation describes technical specifications for adoption of the ITU Recommendation H.320 (an international standard for video-conferencing) functions using the ATM Adaptation Layer 1 (AAL-1). The H.321 terminals conforming to the H.320 specifications interwork with the same type of terminals (i.e., other H.321 terminals) accommodated in B-ISDN, as well as existing H.320 terminals accommodated in a Narrowband Integrated Services Digital Network (N-ISDN).
The H.321 system can provide for synchronization of the H.221 frame based on a common clock signal. However, the H.321 system does not provide (1) a mechanism for compensating the individual audio/video cell losses, (2) a mechanism for synchronization between audio and video signals in view of cell losses and (3) does not specify any particular transport protocols to recover cell losses or errors for data traffic (that may, for example, occur due to header errors in ATM cells or buffer overflows in network congestion).
Typically audio and video traffic may tolerate some cell losses. However, data traffic typically can not tolerate cell losses and has a stringent cell/packet loss requirement. Accordingly, there must be recovery mechanisms for lost data cells. Although timing delays typically allow data cells to be recovered through retransmission, the same is not true of audio or video cells. It is therefore necessary to provide a compensation mechanism for audio and/or video cell losses to improve performance.
An object of the present invention is to provide a method and system for modifying the H.321 specification to provide a method of compensating for lost audio and video cells, a method of maintaining audio and video synchronization, and, specific transport protocols for recovery of data cells, each of which improves performance of H.321-based multimedia conferencing services.